It has been claimed that `reductionism' (broadly understood as a belief in explanation of biological phenomena wholly in physical and chemical terms) has been only of peripheral significance to the development of molecular biology, and even that various forms of non- or anti-reductionism have been important in that development. To examine these claims, this paper investigates the problems of analytical approaches to reductionism and reviews the early scientific and institutional history of molecular biology with respect to the role of reductionist assumptions. It is argued that for complete historical and sociological understanding of the role of philosophical assumptions in molecular biology, reductionism must be considered as a belief system more complex than that implied by any one definition of `reduction' derived from philosophical analysis. It is concluded from the historical review that reductionism was of central significance to the development of molecular biology, since it was central to institutions financially supporting and promoting early molecular biology, and to all the heuristically significant scientific areas of the emerging molecular biological speciality.